There are two answers to that. The first is no. Your code is passed on in sperm and eggs, and the code within them is set at your conception. It’s locked into the egg and locked into the sperm that make you. In the egg’s case that actually occurs inside your grandmother, because your mother’s eggs were developing while she was in utero in her mother. That code is fused together at conception as your genome, and that code should not change over the course of your life. That’s the basic answer.
The caveat to that, which is the second answer, is: yes it will. The reason it will is that its replication isn’t perfect. There are lots of complex mechanisms within the cell which effectively do copy-editing and proof-reading, and if mistakes are introduced during the non-perfect process, they get corrected. When they don’t get corrected, then you potentially run the risk of developing a cancer – that is what a cancer is.
Similarly, your genome could be changed by external factors, such as radiation. If that occurs in the wrong type of gene, that may also result in you getting a cancer. So the code shouldn’t change during your life, but it will. And if you’re unlucky, that will turn into a disease.
“The question is, if your code is unchanging, then why are people different from each other?”
The real question is: if your code is unchanging, then why are people different from each other? That is to do with how the environment interacts with your genome. So nature versus nurture is a false dichotomy – nature via nurture is a better phrase. The gene code itself doesn’t change, but how it interacts with the environment changes profoundly with every action that we do.
There are various mechanisms by which this happens. The one which is incredibly fashionable at the moment is called epigenetics, which is one way in which genes are turned on and off. I’m very sceptical about epigenetics – it is just one of several mechanisms for regulating the unchanging code within our genes.
These things are not mystical in any way. The reason people get healthier when they do more exercise is because of the interaction between your environment and your genetics. All human behaviour is an interaction between the environment and your genes, and the amount of control you have over that relationship is basically controlled by your lifestyle.